The production of the hydrogen for fuel and chemical processing is becoming an increasingly important function in the economy. Until recently, most low-cost hydrogen was produced from fuels, but as the price of fuels increased this method has become less economical. Another method of producing hydrogen is by electrolysis, and recently this method has become more competitive with hydrogen production from fuels even through it is very energy intensive due to the high heat of formation of water. The minimum theoretical voltage for the decomposition of water is 1.23 volts but the actual voltage is at least 1.8 volts because of cell resistance at realistic current densities.
Brecher and Wu U.S. Pat. No. 3,888,750 discloses a process for evolving hydrogen cathodically without the simultaneous evolution of oxygen at the anode. The overall cell reaction for this process is H.sub.2 SO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +H.sub.2 where the voltage for the reaction is 0.17 volts in about 5% sulfuric acid (0.35 V in 50% acid). Since this reaction in theory requires 14% of the energy in the usual electrolysis reaction and yields no less hydrogen per ampere hour, the process is inherently very attractive.
However, a close study of the system shows that the anodic reaction EQU HSO.sub.3.sup.- +H.sub.2 O.fwdarw.HSO.sub.4.sup.- +2H.sup.+ +2e.sup.-
which requires the oxidation of the bisulfite ion, HSO.sub.3.sup.-, to the bisulfate ion, HSO.sub.4.sup.-, may occur with difficulty because the sulfurous acid formed by dissolving sulfur dioxide in an aqueous solution of sulfuric acid is only slightly ionized to form the bisulfite ion in the presence of the stronger, and much more concentrated, sulfuric acid. Thus the bisulfite ion, produced by sulfurous acid, is present at a much lower concentration than the sulfate ion and the bisulfate anion, produced by sulfuric acid. The anode, as the positive electrode, attracts all the anions but does not have a high enough potential to oxidize the sulfate anion and the bisulfate anion. These two ions provide an essentially permanent blanket layer surrounding the anode and block the access of the bisulfite ion to the anode. In addition, since there is no gas evolved at the anode there is no turbulence that would provide fresh access to the anodic surface. These difficulties greatly lower the efficiency of the electrolytic cell.